The object of the work described is a study of the structure of bacterial flagella in the genus Salmonella. Particular emphasis is placed on the relationship between the flagellar antigenic specificity and the primary structure of the flagellar protein, as the system provides an excellent model for a study of the immune response to naturally occurring proteins. The complete amino acid sequence of the flagellar protein isolated from the phase-1 flagella of S. typhimurium will be determined and compared with the flagellar proteins of other flagellar antigens, including mutant forms of the i antigen, to identify those regions of the structure involved in the reaction with specific antibodies. Antigenically active peptide regions, both the peptide already identified and others predicted to be identified in this study, will be studied immunochemically to determine their activity as haptens. Naturally occurring and related peptides will be synthesized to examine the types of antibody present in anti-flagella serum and the size of the antibody combining site. Characterization of such peptides will yield defined antigens for studies on the nature of the immune response.